The present invention relates to a nonaqueous electrolyte battery and a method for manufacturing the same.
In recent years, many portable electronics devices, such as video cameras, electronic notebooks, and laptop computers (portable personal computers), become widespread, and are developed in terms of high performance, miniaturization and weight saving, and portability. Small light batteries with a high capacity have strongly been required as a portable power supply for these electronics devices. On the other hand, demands of secondary batteries, which can be recharged and repeatedly used, have been increasing, instead of primary batteries which are used once and thrown away after discharging.
Conventional secondary batteries which have generally been used are a nickel cadmium (NiCad; NiCd) battery using an alkali electrolytic solution, a lead storage battery and the like. However, the limit of a discharge voltage per cell in these conventional secondary batteries is about 1.2 V. Although various research and developments have been progressing to accomplish further improvements in their discharge capacity, output voltage, and the like, the improvements in the performances seem to reach the limit. It is practically difficult to further improve the energy density, and to accomplish the properties of the miniaturization, the weight saving, and high capacity in the nickel-cadmium battery, the lead storage battery, and the like. Also a self-discharge rate at normal temperatures in the nickel-cadmium battery, lead storage battery, and the like is generally as high as 20% or more in one month. If the batteries are charged once, left during a certain period, and then used again, an apparent discharge amount thereof is decreased due to a natural discharge, and controlling the charge amount becomes complicated, for example, an additional charge is needed before reusing the batteries, which are disadvantages.
Then, nonaqueous electrolyte secondary batteries such as a lithium ion secondary battery with excellent properties have been investigated. In these batteries, a nonaqueous solvent is used as an electrolytic solution, and a light metal such as lithium is used as a negative electrode. This provides the excellent properties: a discharge voltage of 3.7 V or more which is about three or more times of the conventional NiCd battery; a high discharge voltage property resulting in a high energy density; and a low rate of self-discharge. The lithium ion secondary battery is greatly expected to be used as a power supply for electronics devices such as electronic watches used continuously for a long period, backup power supplies for continuously storing data in various memory devices like D-RAM (Random Access Memory), calculators, cameras, and radios in addition to the above portable electronics devices like the laptop computers which requires a high capacity and a long-term charge cycle life.
Moreover, various shapes have been proposed for the lithium ion secondary battery, and thin batteries such as a sheet type battery with a thin large area, and a card type battery with a thin small area are expected to be preferably applicable to various electronics devices with a thinner shape. For example, batteries using a gel-type solid electrolyte in which an electrolyte is impregnated with a matrix polymer, and using a conductive organic macromolecule as a solid electrolyte are suggested for the thin battery. In these batteries using the solid electrolyte, a wound electrode is formed into a flat shape, and covered with an exterior material obtained by composing and laminating, for example, a polyethylene film and aluminum foil. It is expected that this can accomplish a thinner form, weight saving, and a higher energy density compared with the conventional batteries.
On the other hand, it is desired to accomplish much higher energy density in the lithium ion secondary batteries, but this requires further decreasing useless parts which do not contribute to electromotive in the wound electrode.
However, the thin batteries as described above has inherently a small thin outsurface dimension, so it is very difficult to further decrease the useless parts which do not contribute to the electromotive in the conventional nonaqueous electrolyte batteries which comprise the wound electrode composed of a positive electrode, a negative electrode, and a separator which are bonded, or laminated, and spirally wound, which is a problem.
Especially, in the conventional way, the positive electrode, the negative electrode, and the separator are simply laminated, and the separator should be wound many times to form a core of the wound electrode which is positioned approximately in a center part (approximate center part) of the wound electrode, or a core material should be used approximately in the center part. It is actually very difficult or impossible to eliminate these processes. For this reason, the core, which is composed of at least an end of the separator being wound several times, and the core material occupy a space which has no contribution to the electromotive, but they cannot be reduced nor eliminated. This also prevents the improvement in the energy density.
Moreover, the conventional wound electrode, which is formed by simply bonding the positive electrode, the negative electrode, and the separator to form an electrode laminate, and winding the laminate around the core or the core material, has a sloping shape with a cross part of a circle, an ellipse which is more circular, an oval, or a rhombus, and is less likely to be formed into a flat shape. Therefore, the wound electrode should be pressed from the upper and lower surfaces thereof and formed into a flat type one which is applicable to the thin battery. However, the original wound electrode has a shape like a cylinder, so unfortunately, distortion and stress in terms of strength of material easily occur in a process of forming it into the flat type with an even thickness.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a nonaqueous electrolyte battery with a higher energy density in which a useless part which has no contribution to the electromotive is reduced. Moreover, it is another object to provide a method for manufacturing a nonaqueous electrolyte battery which produces easily such a nonaqueous electrolyte battery.